1. Field of the Invention
The present invention relates to a method of manufacturing a circuit board and to a circuit board itself manufactured by said method. Particularly, the present invention provides a method of manufacturing a circuit board using an insulating substrate having a high thermal conductivity, said circuit board being used for mounting a semiconductor element, and the circuit board itself manufactured by the particular method.
2. Description of the Related Art
In the manufacture of a circuit board, metallizing is applied in general to a part or the entire surface region of an insulating substrate formed of, for example, aluminum nitride sintered body, alumina or glass, in an attempt to form a conductor pattern or to achieve a satisfactory bonding with a foreign metal.
Various methods of metallizing the surface of an insulating substrate are known to the art including, for example, a method utilizing a metal having a high melting point, an active metal bonding method, a co-firing method, a method of forming a thick film circuit, a thin film forming method, and a direct bond copper (DBC) method. In the manufacture of a circuit board by the direct bond copper method, a copper plate of a circuit shape is bonded to the surface of the insulating substrate so as to form a conductor pattern. Table 1 shows the resistivity, and ratio of minimum circuit width/distance of the conductor patterns formed by the conventional metallizing methods exemplified above.
TABLE 1 ______________________________________ Resistivity Process Material (m.OMEGA./mm.sup.2) W/D Ratio* ______________________________________ Metallizing of Mo + Mn etc. 10 or less 0.2 mm or high melting point less metal Active metal Ti, Ni 10 or less 0.3 mm or metallizing method less etc. Co-firing W, Mo 10 or less 0.2 mm or method less Thick film Cu less than 0.1 mm or method etc. 3 less Thin film Cu 0.2 .OMEGA./mm.sup.2 0.1 mm or less Au 0.3 .OMEGA./mm.sup.2 0.1 mm or less DBC method Cu; 8.5 .times. 10.sup.-5 .OMEGA./mm.sup.2 0.5 mm ______________________________________
In many cases, a semiconductor element is mounted to the circuit board of the type described above. In recent years, a marked progress is being made in the integration density, allowable operating power, operating speed, etc. of a semiconductor element. In accordance with the progress, the total heat generation and the heat generation per unit area of the semiconductor element are on a sharp increase. To cope with the increased heat generation, an insulating substrate formed of a material having a high thermal conductivity, e.g., aluminum nitride sintered body, or a silicon carbide sintered body, is being developed and put to a practice use. A circuit board comprising an insulating substrate formed of a material having a high thermal conductivity such as an aluminum nitride sintered body or a silicon carbide sintered body produces a prominent effect. Specifically, it is possible to conduct a large current through the semiconductor element mounted on the insulating substrate because the substrate permits an excellent heat dissipation. However, in the case of forming a conductor pattern of a small width and a small distance between adjacent conductive regions on the surface of the insulating substrate by the conventional metallizing methods such as the co-firing method, the conductor pattern exhibits a relatively high resistivity, as shown in Table 1. As a result, various difficulties are brought about in the case of conducting a large current through the semiconductor element mounted on the substrate. For example, in the case of forming a conductor pattern 0.5 mm wide and 20 mm long on the surface of the insulating substrate, the conductor pattern formed by any of the conventional metallizing methods exemplified above exhibits a resistance of 1 to 0.1 .OMEGA.. It follows that a voltage drop of 2 to 20 mV takes place in the case of conducting a current of 20 mA through a signal line of 2 V. The voltage drop noted above is as large as 0.1 to 1% of the signal line voltage. Also, when it comes to a circuit board having a power transistor of 500 V mounted on a conductor pattern having a width of 5 mm and a length of 20 mm, a power of 0.1 to 1 W is consumed uselessly in the case of conducting a current of 10 A.
It is certainly possible to lower the resistivity of the conductor pattern when it comes to a circuit board manufactured by employing the direct bond copper method, or a circuit board prepared by metallizing the surface of an insulating substrate by means of the active metal bonding method, followed by forming a conductor patter by bonding a metal plate or foil of a circuit shape on the metallized surface of the substrate by means of soldering or brazing. Naturally, it is possible to conduct a large current through the semiconductor element mounted on such a substrate. However, the particular methods exemplified above require the step of processing in advance a copper plate in a circuit shape, leading to a complex manufacturing process. Also, it is difficult to form a conductor pattern of a high dimensional accuracy. In addition, problems such as a thermal expansion and a thermal deformation are brought about in the bonding step. Under the circumstances, it is difficult to form a conductor pattern on the surface of an insulating substrate with a high density, when it comes to the conventional metallizing methods. As a matter of fact, it is substantially impossible to make the conductor pattern finer than 0.5 mm in both the width and distance in the conventional metallizing methods.
Also proposed is a method in which a copper plate is bonded to a substrate surface by the direct bond copper method, followed by applying a chemical etching or electrolytic etching to the copper plate so as to form a fine conductor pattern. However, this technique is defective in that much time is required for the corrosion of the insulating substrate and for the aftertreatment.
An additional difficulty remains unsolved in the case of a circuit board prepared by the direct bond copper method or a circuit board prepared by bonding a metal plate or foil of a circuit shape to the surface of an insulating substrate metallized by the active metal bonding method. Specifically, the conductor pattern of such a circuit board has a relatively large thickness, i.e., 0.1 mm or more, with the result that the circuit board bears a residual thermal stress derived from the difference in the thermal expansion coefficient between the conductor pattern and the insulating substrate. As a result, cracking tends to take place at the interface between the insulating substrate and the conductor pattern, particularly, in the insulating substrate in the vicinity of the edge portion of the conductor pattern or in the bonding interface. To be more specific, the thermal stress applied to the interfacial region between the insulating substrate and the conductor pattern is alleviated by the temperature change in the step of bonding a metal member such as a semiconductor element, a lead wire or a seal ring to the conductor pattern or by the thermal history during the use of the semiconductor element. As a result, cracking tends to take place at the bonding interface. A similar problem also takes place in the step of applying a reliability accelerating test or TCT (thermal cycle test) to the semiconductor element mounted to the circuit board. It has been confirmed that the difficulty becomes serious with increase in the thickness of the conductor pattern. In the case of a circuit board utilizing a combination of, for example, a copper plate and an aluminum nitride substrate, it was practically impossible in the conventional technique to make the copper plate thicker than 0.3 mm.
On the other hand, Published Unexamined Japanese Patent Application No. 63-179734 discloses a circuit board comprising a substrate formed of, for example, an aluminum nitride sintered body having a high thermal conductivity, an active metal layer formed on the surface of the substrate, and a copper-based alloy member formed on the active metal member and having a porosity of at most 10% by volume. In the circuit board disclosed in this prior art, however, it is necessary to process in advance the copper-based alloy plate in a circuit shape, leading to a complex manufacturing process of the circuit board. In addition, it is difficult to form a circuit pattern of a high dimensional accuracy.